[DOCUMENTATION] Update Documentation to Symfony 4 standards

[brianfreytag]

Given the updated directory structure, the packages-level configurations, and the bundle registration not being in AppKernel any longer, I propose we update the 2.0.0 Documentation to reflect these kinds of changes. We can have a <= 3.4 version of the documentation available as well (or inline, depending on preference).

If I get a few thumbs up, I can work on that.

[github-actions]

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